1. Field of the Invention
The present invention relates to a method of manufacturing a grating which modulates refractive index in an optical waveguide such as an optical fiber or a planar lightwave circuit and, more particularly, to a method of manufacturing a grating of a grating device such as a band-pass filter or a dispersion equalization device used in an optical communication system which need precise characteristics.
2. Description of the Prior Art
In an expected high density wavelength division multiplex transmission system (hereinafter referred to as DWDM transmission system) having a wavelength interval of 50 GHz (wavelength: 0.4 nm) or 25 GHz (wavelength: 0.2 nm) expected to be realized, a band-pass filter obtained by disposing a grating in an optical waveguide such as an optical fiber or a planar lightwave circuit (hereinafter referred to as PLC) is a necessary device. In a future ultra-high speed transmission system having a bit rate of not less than 10 Gbit/s, or not less than 40 Gbit/s, a dispersion equalization device obtained by disposing a grating in an optical waveguide is a necessary device.
The grating device such as a band-pass filter or a dispersion equalization device can be manufactured such that, for example, the interference fringes of an ultraviolet laser beam are irradiated on an optical waveguide such as an optical fiber or a planar lightwave circuit (PLC) obtained by adding Ge to a core consisting of silica. In addition, more specifically, the grating device is manufactured by the following operations. An optical waveguide is left in high-pressure hydrogen of several 10 to several 100 atms for several days to several weeks to fill hydrogen in the optical waveguide, and a change in refractive index by ultraviolet irradiation is sensitized. The interference fringes of ultraviolet laser beams split into two-beam of light by a phase mask or a half-mirror, and a change in refractive index depending on the interference fringes is formed. The formation of the change in refractive index depending on the interference fringes is called modulation, and the magnitude of the change is called the degree of refractive index modulation. When a grating pitch of the grating formed in the optical waveguide is represented by xcex9, and when an equivalent refractive index of the optical waveguide is represented by Neff, of light components being incident on the optical waveguide, a light component having a wavelength xcexB which satisfies the following Equation 1 causes Bragg reflection and is reflected on the incident side.
[Equation 1]
xcexB=2xc2x7Neffxc2x7xcex9xe2x80x83xe2x80x83(1) 
Note that an equivalent refractive index is an equivalent refractive index which is received by a light component propagated through the optical waveguide, is determined by an interactive operation between a core and a cladding, and is also called an effective refractive index or a valid refractive index.
When a wavelength xcexB which satisfies the relationship of Equation 1 is set to be constant over the entire grating, only a light component having a specific wavelength can be efficiently reflected. For this reason, a band-pass filter having a band-pass characteristic which is considerably sharp can be obtained. On the other hand, the grating pitch xcex9 or the equivalent refractive index Neff of the grating are changed in a propagation direction to form a chirp grating the Bragg wavelength xcexB is changed depending on the position of the grating, so that a dispersion equalization device such as a dispersion compensator or a dispersion slope compensator can be obtained.
In order to obtain a characteristic used in an application of the band-pass filter or a dispersion equalization device, high precision is required to manufacture the grating. More specifically, in a band having a wavelength of 1.55 xcexcm used in an optical communication system, a grating pitch is about 500 nm, and the grating should be uniformly manufactured over a length from about several mm to about 100 mm of the grating. In addition, since the degree of refractive index modulation or the equivalent refractive index Neff also changes by an irradiation amount of an ultraviolet laser beam, the ultraviolet laser beam should be irradiated at a high precision over the entire length of the grating. The error of the grating pitch from the design is called a phase error, and an error of the degree of refractive index modulation or the equivalent refractive index Neff is called an amplitude error. These errors cause degradation of an amount of out-of-band attenuation in a band-pass filter, and cause a ripple of a group delay time characteristic, i.e., a group delay ripple in an dispersion equalization device. This fact is described in [Richardo Feced, et al., xe2x80x9cEffect of Random Phase and Amplitude Errors in Optical Fiber Bragg Gratingsxe2x80x9d, Journal of Lightwave Technology, Vol. 18, No. 1, pp. 90-101, January, 2000, issued by IEEE].
As methods of manufacturing gratings the errors of which are reduced, several methods are proposed. For example, in a method of manufacturing an optical waveguide diffraction grating disclosed in Japanese Laid-Open Patent Publication No. 8-286066, as shown in the perspective view in FIG. 17, fluorescence having a predetermined wavelength (about 240 nm) irradiated to detect fluorescence generated when the grating is formed, and alignment is performed such that an amount of received fluorescence is maximum. More specifically, when an ultraviolet laser beam having a wavelength of about 240 nm is irradiated on an optical fiber, fluorescence having wavelength of 350 to 550 nm is generated by the core of the optical fiber. A part of the generated fluorescence is propagated through the optical fiber to reach a detector 8. An amount of fluorescence received by the detector 8 is adjusted to maximum, so that a laser beam irradiated on the core of the optical fiber 1 is maximum. As an ultraviolet laser having a wavelength of 240 nm, a KrF excimer laser (wavelength of 248 nm) and a second harmonic (wavelength of 244 nm) of an argon laser are known.
In [Komukai Tetsuro, et al., xe2x80x9cExamination of Cause of Generation of Group Delay Ripple in Chirp Fiber Gratingxe2x80x9d, Technical Report of the Institute of Electronics, Information and Communication Engineers OFT2000-49, pp. 31-35, issued by, a corporation, the Institute of Electronics, Information and Communication Engineers], the following is described. That is, the position of an optical fiber is controlled such that an ultraviolet laser beam is uniformly irradiated by always monitoring fluorescence while scanning an ultraviolet laser beam having a wavelength of 244 nm which is a second harmonic of an argon laser in the direction of the optical axis of the optical fiber. In addition, many causes of group delay ripples generated by chirp gratings used as dispersion equalization device exist in processes in manufacturing the gratings, the following causes will be described:
(1) Fluctuation of the power or mode of an ultraviolet laser beam to be irradiated (amplitude error).
(2) Fluctuation of the composition of the core of an optical waveguide such as an optical fiber in the longitudinal direction.
(3) Incompletion of apodization of a chirp grating.
(4) A positional error between a phase mask and an optical waveguide caused by mechanical vibration (phase error).
(5) Incompletion of positional control of an optical waveguide and laser beam irradiation (amplitude error).
(6) Insufficiency of washing of an optical waveguide (amplitude error).
(7) Incompletion of a phase mask such as a stitching error (amplitude error and phase error).
On the other hand, in a method of manufacturing a grating disclosed in Japanese Laid-Open Patent Publication No. 10-90545, as shown in FIG. 18, heat generated when a KrF excimer laser beam having a wavelength of 248 nm and serving as an ultraviolet laser beam is irradiated on an optical waveguide formed in a PLC is radiated through a heat radiator. In this case, when the ultraviolet laser beam is irradiated for several minutes to several ten minutes to manufacture a grating, a part of the ultraviolet laser beam reaching a substrate through a cladding and a core is absorbed to heat the substrate. Heat generated at this time is radiated from a PLC 1 through a heat radiator 5. In this manner, an increase in temperature of the entire PLC 1 is suppressed to xc2x110xc2x0 C. or less, and the grating pitch is suppressed from being changed by thermal expansion of the PLC 1.
However, even though the various error generation causes described above are prevented, a phase error is inevitably generated. More specifically, when an ultraviolet laser beam having a beam width of several mm or less is scanned in the direction of the optical axis of the optical waveguide to manufacture a grating, an error of a grating pitch caused by thermal expansion by local heating, i.e., distortion is generated in the optical waveguide. A phase error is generated by the distortion of the grating pitch. As a cause of the distortion of the grating pitch, the following may be considered. In general, since an optical waveguide consists of silica (SiO2) as a main component, the thermal expansion coefficient of the optical waveguide is small, the optical waveguide has a thermal expansion coefficient of about 10xe2x88x926. For this reason, for example, when an ultraviolet laser beam having a beam width of 1 mm is irradiated on a region of an optical waveguide having a length of 1 mm to increase the temperature of the region by several degrees centigrade, the region of 1 mm thermally expands by about several nm to push another region out every several nm. Since the ultraviolet laser beam is scanned along the optical axis of the optical waveguide, the irradiated region thermal expands by several nm when another region of the optical waveguide is irradiated, and another part is pushed out every several nm. In this manner, even though a local temperature increases by only several degrees centigrade by the irradiation of the ultraviolet laser beam, a fluctuation of about plus or minus several nm of the grating pitch occurs. Since the grating pitch is about 500 nm, for example, even though the fluctuation of about several nm occurs, the grating is considerably influenced, and the fluctuation causes a phase error to generate a group delay ripple. This group delay ripple adversely affects not only a chirp grating used in a dispersion equalization device, but also a uniform grating used in a band-pass filter.
In addition, in general, as described in a method of manufacturing an optical waveguide diffraction grating described in Japanese Laid-Open Patent Publication No. 8-286066, an ultraviolet laser beam having a wavelength of about 240 nm is used. As the ultraviolet laser beam, a KrF excimer laser (wavelength of 248 nm) and a second harmonic (wavelength of 244 nm) of an argon laser are known. However, since the time and space stabilities of the coherence and energy of the KrF excimer laser are not good, the KrF excimer laser is not stable for manufacturing a precise grating. In addition, although the second harmonic of the argon laser has high coherence, the second harmonic is continuously oscillated. For this reason, efficiency of change in refractive index is poor, and a large energy density is required to obtain a sufficient degree of refractive index modulation. Therefore, an argon laser beam is converged to be irradiated on the optical waveguide, an irradiation amount of the ultraviolet laser beam considerably varies because of a small positional error, and an amplitude error is generated. In addition, although the laser beam is irradiated on the optical waveguide with a large energy density by convergence, local thermal expansion of the optical waveguide is not considered. For this reason, slight fluctuation occurs in the grating pitch, and a phase error is also generated.
The conventional technique described above is examined, a method of manufacturing a grating described in Japanese Laid-Open Patent Publication No. 10-90545 is to keep the temperature of an entire PLC in which an optical waveguide is formed constant. Therefore, when an ultraviolet laser beam is uniformly irradiated on the entire optical waveguide, no problem is posed. However, local thermal expansion caused by local heating when an ultraviolet laser beam having a beam width of several mm or less is scanned to be irradiated on the optical waveguide is not considered. More specifically, even though a heat radiator or a heat sink is arranged on the entire PLC, or cooling is forcibly performed, the optical waveguide at a portion on which the ultraviolet laser beam is irradiated locally thermally expands. In other words, for example, when no heat radiator or the like is arranged, the temperature of the portion on which the ultraviolet laser beam is irradiated with reference to the temperature of the peripheral portion can be kept constant on the average even if a heat radiator or a cooling mechanism is disposed on the portion. This configuration is not sufficient to cancel a difference between temperatures of the peripheral portion of the portion on which the ultraviolet laser beam is irradiated. Therefore, the slight fluctuation occurs in the grating pitch as described above, and a phase error is generated.
It is the first object of the present invention to provide a method of manufacturing a grating in which local thermal expansion occurring, when an ultraviolet laser beam is scanned on an optical waveguide to manufacture a grating, is suppressed to reduce a phase error. It is the second object of the present invention to provide a method of manufacturing a grating in which an amplitude error is reduced.
In accordance with one aspect of the present invention, there is provided a method of manufacturing a grating in an optical waveguide. The optical waveguide includes a core and a cladding covering the core. The core is made of a material having the refractive index that is changeable by irradiation of radiation (e.g. ultraviolet rays). The method includes the steps of providing the optical waveguide and scanning laser beam along an optical axis of the optical waveguide to form modulation of refractive index of the radiation (e.g. ultraviolet rays) in the core. In addition, on the step of scanning the laser beam, in the core, an irradiation range of the rays is controlled, and the laser beam is scanned a plurality of times. Then predetermined distribution of irradiation amount is obtained in a direction of the optical axis of the grating. Preferably, the laser beam may be ultraviolet laser beam.
In another aspect of the present invention, there is provided a method of manufacturing a grating in an optical waveguide. The optical waveguide includes a core and a cladding covering the core. The core is made of a material having the refractive index that is changeable by irradiation of radiation (e.g. ultraviolet rays). The method includes the steps of providing the optical waveguide and scanning laser beam along the optical axis of the optical waveguide to form modulation of refractive index of radiation (e.g. ultraviolet rays) in the core. In addition, on the step of scanning, the laser beam is scanned along the optical axis of the optical waveguide at the scanning speed not lower than the predetermined scanning speed. The scanning speed may be defined by energy density E per unit time of the laser beam and beam diameter B.
In a further aspect of the invention, the laser beam is scanned at the scanning speed not lower than the scanning speed:
B2/(85xc2x7Exe2x88x921.2)(mm/second) 
defined by the energy density E (W/cm2) per unit time and the beam diameter B (mm).
In a still further aspect of the present invention, on the step of scanning, an irradiation range of radiation (e.g. ultraviolet rays) is controlled, and the laser beam is scanned a plurality of times. Therefore, a predetermined distribution of irradiation amount is obtained in the direction of the optical axis of the grating in the core.
In a yet further aspect of the present invention, the optical waveguide is arranged on a thermal conductive substrate.
In a yet further aspect of the present invention, the laser beam is scanned along the optical axis of the optical waveguide at scanning speed not lower than the predetermined scanning speed. The scanning speed may be defined by energy density E per unit time of the laser beam, beam diameter B, and thermal conductivity k of the thermal conductive substrate.
In a yet further aspect of the present invention, the laser beam is scanned at scanning speed not lower than the scanning speed:
B2/[(115xc2x7k)0.5xc2x7Exe2x88x921.2)(mm/second) 
defined by the energy density E (W/cm2) per unit time, the beam diameter B (mm), and the thermal conductivity k (J/(mxc2x7k)) of the thermal conductive substrate.
Preferably, the laser beam may be pulse laser beam. More preferably, the pulse laser beam may be ultraviolet pulse laser beam.
Also, the light source of the pulse laser beam may be a semiconductor light source.
In a yet further aspect of the present invention, the pulse laser beam has an energy density not lower than energy density at a change point where an inclination of a refractive index increase coefficient to energy density per pulse changes.
In a yet further aspect of the present invention, the optical waveguide is arranged on a mirror surface substrate for reflecting a laser beam. In addition, a reflected beam reflected by the mirror surface substrate, when the laser beam is irradiated on the optical waveguide, is monitored to adjust relative positions of an irradiation position of the laser beam and the optical waveguide.
In a yet further aspect of the present invention, in the optical waveguide, the laser beam is scanned while locally cooling a portion on which the laser beam is irradiated.
The optical waveguide may be locally cooled by air.
According to a method of manufacturing a grating in an optical waveguide of the present invention, an irradiation range of laser beam scanned along the longitudinal direction of an optical waveguide is controlled, and scanning is performed a plurality of times, so that a predetermined distribution of irradiation amount can be obtained in the direction of the optical axis of the grating. In this manner, apodization in which a distribution of a predetermined degree of refractive index modulation is formed can be performed. In addition, since a predetermined distribution of irradiation amount can be obtained by performing scanning a plurality of times, an irradiation amount per scanning can be reduced, and distortion of a grating pitch can be suppressed by suppressing local thermal expansion.
According to the method of manufacturing a grating of the present invention, a laser beam is scanned at scanning speed not lower than the scanning speed defined by energy density E per unit time of the laser beam and beam diameter B. For this reason, local thermal expansion of an optical waveguide can be suppressed, and distortion of a grating pitch can be suppressed by reducing a phase error.
According to the method of manufacturing a grating of the present invention, a laser beam is scanned at a scanning speed not lower than a scanning speed: B2/(85xc2x7Exe2x88x921.2)(mm/second) defined by energy density E (unit: W/cm2) per unit time of the laser beam and beam diameter B (unit: mm). For this reason, local thermal expansion of an optical waveguide can be suppressed, and distortion of a grating pitch can be suppressed by reducing phase error.
Furthermore, according to the method of manufacturing a grating of the present invention, an irradiation range of a laser beam scanned along the longitudinal direction of an optical waveguide is controlled, and scanning is performed a plurality of times, so that a predetermined distribution of irradiation amount can be obtained in the direction of the optical axis of the grating. In this manner, apodization in which a distribution of a predetermined degree of refractive index modulation is formed can be performed. In addition, since predetermined distribution of irradiation amount can be obtained by performing scanning a plurality of times, an irradiation amount per scanning can be reduced, and distortion of a grating pitch can be suppressed by suppressing local thermal expansion.
Still more, according to the method of manufacturing a grating of the present invention, since an optical waveguide is arranged on a thermal conductive substrate, local heat generated by irradiation of a laser beam is diffused through the thermal conductive substrate, and local thermal expansion caused by local heating of the optical waveguide is suppressed, so that distortion of a grating pitch can be suppressed by reducing a phase error.
According to the method of manufacturing a grating of the present invention, a laser beam is scanned at a scanning speed not lower than a scanning speed defined by an energy density E per unit time of the laser beam, a beam diameter B, and a thermal conductivity k of a thermal conductive substrate. For this reason, local thermal expansion of the optical waveguide can be suppressed, and distortion of a grating pitch can be suppressed by reducing a phase error.
Furthermore, according to the method of manufacturing a grating of the present invention, the laser beam is scanned at a scanning speed not lower than a scanning speed: B2/[(115xc2x7k)0.5xc2x7Exe2x88x921.2] (mm/second) defined by the energy density E (unit: W/cm2) per unit time, the beam diameter B (unit: mm), and the thermal conductivity k (W/(mxc2x7k)) of the thermal conductive substrate. For this reason, local thermal expansion of an optical waveguide can be suppressed, and distortion of a grating pitch can be suppressed by reducing a phase error.
Still more, according to the method of manufacturing a grating of the present invention, since an pulse laser beam is used as a laser beam, a high energy density can be obtained, and a change in refractive index can be efficiently caused.
According to the method of manufacturing a grating of the present invention, since an pulse laser beam obtained by a semiconductor light source is used as a laser beam, a distribution of irradiation amount of the laser beam to an optical waveguide caused by time and space changes in intensity of the laser beam has slight fluctuation. For this reason, an amplitude error can be reduced, and a change in refractive index caused by irradiation can be efficiently performed.
Furthermore, according to the method of manufacturing a grating of the present invention, the pulse laser beam has an energy density not lower than an energy density at a change point where an inclination of a refractive index increase coefficient to an energy density per pulse changes. For this reason, a change in refractive index caused by irradiation can be efficiently performed.
Still more, according to the method of manufacturing a grating of the present invention, an optical waveguide is arranged on a mirror surface substrate, and a reflected light component from the mirror surface substrate is monitored to adjust the relative positions of the irradiation position of a laser beam and the optical waveguide. For this reason, the optical waveguide can be irradiated in a range in which the laser beam has a high energy density and a small change in energy. In this manner, a change in refractive index efficiently caused by reducing an amplitude error.
According to the method of manufacturing a grating, since a laser beam is irradiated while locally cooling a portion on which the laser beam is irradiated in the optical waveguide, local thermal expansion caused by local heating of the optical waveguide is suppressed, and distortion of a grating pitch can be suppressed by reducing a phase error.
Furthermore, according to the method of manufacturing a grating of the present invention, since laser beam is irradiated while cooling an optical waveguide by an air cooling method, local thermal expansion caused by local heating of the optical waveguide is suppressed, and distortion of a grating pitch can be suppressed by reducing a phase error.